This invention generally relates to color picture tubes and, more particularly, a method and apparatus for fabricating tension focus masks for color picture tubes.
A color picture tube includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate of the tube and is made up of an array of elements of three different color emitting phosphors. A color selection electrode mask, otherwise known as a shadow mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of material, such as steel, that is contoured to somewhat parallel the inner surface of the tube faceplate. A shadow mask may be either formed or tensioned. There are three types of tension mask systems: (1) strand tension mask; (2) tie bar tension mask; and (3) tension focus mask. A tension focus mask comprises two sets of conductive members that are perpendicular to each other and separated by an insulator. The two sets of members are held at different voltages; thus, creating electron focusing lenses within each rectangular space encompassed by two adjacent mask strands and two adjacent crosswires. A tension focus mask has at least one of the sets of conductive members under tension. Generally, in a tension focus mask, a vertical set of conductive members or mask strands is under tension and a horizontal set of conductive members or crosswires overlies the mask strands.
In assembling a tension focus mask, it is required to assemble the crosswires and mask strands with a high degree of accuracy to achieve consistent spacing between the mask strands and between the crosswires. If the spacing between vertical mask strands is consistent, then the tension focus mask will not exhibit macroscopic streaks, and as such, those macroscopic streaks will not be printed into the matrix and screening array.
The present invention provides a method and apparatus for assembling a tension focus mask and maintaining uniform spacing between the vertical members, or mask strands, of the mask. The method includes providing a tension focus mask comprising vertical mask strands which are held in tension. A permanent adhesive is sprayed on the screen-side of the mask strands followed by the attachment of horizontal guide members to the gun-side of the mask strands with the use of a non-permanent adhesive. Next, permanent horizontal crosswires are applied to the screen-side of the mask strands; subsequently, the mask assembly is placed into an oven and heated to permanently adhere the permanent crosswires to the mask strands and also to remove the temporary guide members from the mask strands as the non-permanent adhesive volatilizes away.